1. Technical Field
The present invention relates to a film-forming ink, a film-forming method, a liquid droplet discharging device, a method for preparing a light-emitting element, a light-emitting element, a light-emitting device, and an electronic apparatus.
2. Related Art
Organic electroluminescence elements (organic EL elements) are light-emitting elements each composed of a positive electrode, a negative electrode, and at least one light-emitting organic layer (light-emitting layer) interposed between them. In this light-emitting element, by applying an electric field between the negative electrode and the positive electrode, electrons and holes are injected from the negative electrode and the positive electrode, respectively, to the light-emitting layer, where they are recombined to form excitons in the light-emitting layer, and then when the excitons return to the ground state, energy is emitted in the form of light.
Generally, in the organic EL element, a hole injecting layer is provided on the positive electrode, and a hole transporting layer or a light-emitting layer is provided on the hole injecting layer.
As a method for forming these layers (film-forming method), there is known a method involving a film-forming ink in which film-forming materials are dissolved or dispersed (see, for example, JP-A-2008-77958).
Since this film-forming method is capable of patterning without the use of a photolithographic method or a vacuum deposition method using a mask, there is an advantage that the preparation process is simple and at the same time, the amount of the raw material to be used may be reduced.
Generally, for the constituent materials of each layer in an organic EL element, a π-conjugated compound is used.
However, a film-forming ink in the related art has had a problem that the transporting ability of the carrier (holes or electrons) of the resulting film increases too much. It is thought that this problem is caused by a fact that π-conjugated compounds are easily aggregated with one other by the effect of π-π stacking in a solvent or a dispersion medium. Further, if the transporting ability of the carrier of the layer constituting the organic EL element is too high, the transporting balance of the holes and the electrons is deteriorated, leading to reduction of the light-emitting efficiency.
Accordingly, if a film is formed by a liquid droplet discharging method using the film-forming ink, there is no action of a force for prevention of aggregation among the π-conjugated compounds, unlike a spin coating method, and thus, the π-conjugated compounds are easily aggregated with one another and the problems as described above become remarkable.